加载表单后如何运行函数 Kivy

I have a kivy program in python which has a textbox and a few buttons. I have written the ui in kivy language and I need to run a function which updates the text box and waits for the user to press a button. Is there an on_load property I can use or some sort of thing to run this function when all the widgets have been loaded. The .kv file:

<MainGui>:
   id: layout1
   orientation: 'horizontal'
   #I would like for some sort of event like below to run my function:
   on_load: root.myfunction()

解决方案

from kivy.app           import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label     import Label

class DemoApp(App):

    def build(self):
        self.layout = layout = BoxLayout()
        layout.label = Label(text="INITIAL TEXT")
        layout.add_widget(layout.label)
        return(self.layout)

    def on_start(self, **kwargs):
        self.layout.label.text = "APP LOADED"

DemoApp().run()